The state-of-the-art method for electrolyzing an alkali metal halide, especially sodium chloride (NaCl) or potassium (KCl), is to use a fluorinated membrane to separate the anolyte and catholyte compartments. The membrane permits the alkali metal cation to pass through to the catholyte, but severely restricts the undesirable passage of hydroxyl ion from the catholyte to the anolyte. To make membrane electrolysis attractive, the power consumption should be minimized, which means that the current efficiency should be maximized and the cell voltage (or resistance) should be minimized.
Many efforts have been made to improve the performance of these membranes by a wide variety of treatments. Most of them have been aimed at higher current efficiency or lower power consumption. However, it is also desirable to obtain lower voltage. Any voltage over that needed to electrolyze brine is lost as heat and this represents a waste of electric power. Excessive heat production can limit electrolyzer productivity by raising cell temperatures and increasing gas volume. Also, the rectifiers used in chloralkali plants are rated for power, which is the product of voltage and amperage. At higher voltages, less amperage can be supplied, reducing the productivity of the electrolyzers.
The art which is believed to be closest to the present invention U.S. Pat. No. 4,439,292 (Klotz et al., Bayer). These inventors reduced cell voltage by treating fluorinated carboxyl membranes with corona discharge, a process in which an electrical discharge or spark is passed from one electrode to another through a gas and through the sample to be treated. Klotz et al. found that heating damaged the membrane. They chose to keep the temperature below 80.degree. C., preferably below +22.degree. C., and most preferably below -20.degree. C. To prevent heat damage, Klotz et al. exposed the sample to corona discharge 1000-5000 times, with cooling between exposures. Clearly, the need to use thousands of exposures is a severe limitation to the usefulness of this process.